1. Field of the Invention
The present invention relates a scanning line driving circuit for an active matrix and a structure of an image display device, and more particularly to a scanning line driving circuit for an active matrix and an image display device preferably applied to a liquid crystal display device that employs an amorphous silicon thin film transistor in a scanning line driving circuit having a contact hole.
2. Description of the Related Art
In an image display panel of a liquid crystal display device having a scanning line driving circuit for an active matrix, as a gate line driving circuit (a scanning line driving circuit) for scanning the display panel, a shift register can be used that carries out a shift operation of making a round in one frame period of a display signal. The shift register is desirably formed with only the same conductive type field effect transistors in order to reduce the number of processes in a production process of the display device.
The liquid crystal display device in which the shift register of the gate line driving circuit is formed with the amorphous silicon thin film transistors (refer it to as “a-Si TFT”, hereinafter) is easy in its enlargement of an area and high in its productivity, and is used for a screen of, for instance, a notebook type PC, a personal digital assistant (PDA), a multi-media player (PMP), a simple car navigation system (PND: Personal Navigation Device) or the like (see Jin Young Choi, Jin Jeon, Jong Heon Han, Seob Shin, Se Chun Oh, Jun Ho Song, Kee Han Uh, and Hyung Guel Kim, “A Compact and Cost-efficient TFT-LCD through the Triple-Gate Pixel Structure”, pp. 274 to 276, SID '06 DIGEST).
Further, in order to make the frame of an active matrix liquid crystal display panel narrow, the gate line driving circuit needs to be arranged in the vicinity of a seal material formed so as to surround the peripheral edge of the liquid crystal display panel or a part of the driving circuit needs to be arranged under the seal material.
The gate line driving circuit has many contact holes for connecting different kinds of metal wirings. The contact holes serve to electrically connect together a first metal thin film and a second metal thin film formed in different layers. The contact hole is formed on the first metal thin film and the contact hole is formed on the second metal thin film, and both the contact holes are bridged by an electrically conductive film between them.
As a material of the electrically conductive film for connecting together the metal wirings opened by the contact holes, a transparent electrically conductive film such as ITO is ordinarily used. Since the ITO film is inferior in its coverage property (see JP-A-11-281992), the metal thin film is not covered with the transparent electrically conductive film in a part of the contact holes, so that the metal thin film is exposed in some parts.
Since the part near the inner part of the seal material or under the seal material is liable to receive an influence of water, impurities or the like, the metal thin film comes into contact with the water, the impurities or the like in the part inferior in the coverage property due to the contact hole formed in the vicinity of the inner part of the seal material or under the seal material. Then, a wiring, a terminal, an electrode, etc. made of the metal thin film are corroded due to an oxidation (a corrosion phenomenon of the metal wiring or the like near the contact hole is called a contact hole corrosion, hereinafter).
Further, the inferior coverage property of the ITO film arises with an extremely high frequency especially when a process is employed in which the ITO film is formed with an amorphous material and then crystallized. Ordinarily, for a pattern process of the ITO film, a wet etching process by chemicals is frequently used. In the case of a crystalline ITO film, as the chemicals used in the wet etching process, a strong acid composed of aqueous solution of hydrochloric acid+nitrate needs to be used. In such a case, when the metal thin films such as Al, Ag or Mo coexist as a gate signal line, a source signal line or a reflection electrode, there is a fear that the metal thin films are corroded and disconnected during the wet etching process of the ITO film.
In contrast, in the case of an amorphous ITO film, the wet etching process can be carried out by a weak acid such as aqueous solution of oxalate. Therefore, even when the metal thin films such as Al, Ag or Mo coexist, the metal thin films are not corroded and disconnected. Accordingly, a process is preferably used in which the ITO film is initially formed under an amorphous state, a pattern process is carried out by using etching solution composed of oxalic acid, then, the ITO film is crystallized by using, for instance, a heating unit and finally chemically stabilized.
However, when the phase of the ITO film changes from the amorphous state to the crystalline state, a volumetric shrinkage (a distance between crystalline atoms is narrowed) arises due to the change of an irregular arrangement structure to a regular arrangement of the atoms. Therefore, since a tensile stress from a substrate is applied to the ITO film, a stage cut and disconnection of the ITO film is apt to arise especially in a step part such as the contact hole. As described above, it is preferable in view of etching to cover the contact hole with the amorphous ITO film, however, the covering or coating property of the ITO film is not good. Accordingly, the contact hole corrosion may be occasionally caused owing to the entry of water or impurities.
Specially, in the case of the gate line driving circuit formed with the a-si TFT, for instance, when it is assumed that a High voltage is 24V and a Low voltage is −6V, a potential difference is 30V so that a signal amplitude in this circuit is very large. When the gate driving circuit is operated under an environment of high temperature and high humidity, a serious problem arises that what is called an electrolysis reaction occurs due to the potential difference and the metal wiring exposed due to the inferior coverage property of the contact hole is corroded as a result of application of a high potential thereto.